It is desirable to have a power efficient amplifier that produces a voltage to current transformation. The voltage to current transformation is desired in such applications as motor control where the current through the motor coil produces a proportional force. One approach to the design of a power efficient amplifier is illustrated in prior art FIG. 1 which illustrates a closed loop current amplifier. In the closed loop current amplifier, an input voltage, V.sub.in, is applied to the input of the circuit. An amplifier in conjunction with a power stage serves to force the output voltage, V.sub.out, equal to the input voltage, V.sub.in. The appearance of V.sub.out across the sampling resistor, R.sub.s, is associated with a current I.sub.out that flows through the sampling resistor, R.sub.s. Because of the properties of the amplifier, its input current I.sub.1 and I.sub.2 are very close to zero, so that practically all of the current I.sub.out is supplied by the power stage. The current through the load, I.sub.load, is then approximately equal to I.sub.out and produces a voltage at the output of the power stage of V.sub.p which is equal to the product of I.sub.out multiplied by the sum of Z.sub.load and R.sub.s. The magnitude of V.sub.p must be less than or equal to the magnitude of the supply voltage V.sub.s for the circuit to operate properly. The power required for operation is the product of the supply voltage and output current, assuming that the amplifier draws negligible current.
A problem with such an amplifier is that it must be constructed to deliver the maximum required power, which means that it will be inefficient at some loads. Also, building for the peak demand is necessarily more expensive than building for lesser current demands. These disadvantages are evident in applications where the nominal value of the load current I.sub.1, I.sub.1n is much less than its maximum value, I.sub.1m. The power supply voltage must be chosen for proper operation at I.sub.1m. However it is clear that during nominal operation, the power requirement could be reduced by a factor of I.sub.1m /I.sub.1n if the power supply voltage could be changed to that determined by I.sub.1n. Accordingly, it would be highly desirable to have a power supply that operates on the closed loop principle. It is desirable to have a power supply which adjusts the output in response to the demands made for power.